Engineered nanomaterial (ENM) production has expanded significantly in the last decade, with sales increased from $0.4 billion (USD) in 2005 to $1.4 billion (USD) in 2010. See Frost & Sullivan, Nanomaterials—Strategic Portfolio Management (Technical Insights), 2010. Sales of nanocomposites, produced by the addition of ENMs to polymeric matrices, are estimated to reach $2.4 billion (USD) by 2016. See BCC Research NAN021E—Global Markets for Nanocomposites, Nanoparticles, Nanoclays, and Nanotubes. As applications for ENMs continue to expand, there is increasing concern about potential health and environmental risks associated with exposure to nanoparticles from ENMs. The nanoparticles, due to their small size, high surface area and surface reactivity, have the potential to induce cytotoxic effects, see for example, Magrez, A.; Kasas, S.; Salicio, V.; Pasquier, N.; Seo, J. W.; Celio, M.; Catsicas, S.; Schwaller, B.; Forró, L. Nano Lett. 2006, 6, 1121, as well as genotoxic effects, inflammation and even cancer. See for example, Magrez, A.; Kasas, S.; Salicio, V.; Pasquier, N.; Seo, J. W.; Celio, M.; Catsicas, S.; Schwaller, B.; Forró, L. Nano Lett. 2006, 6, 1121 and Savolainen, K.; Alenius, H.; Norppa, H.; Pylkkänen, L.; Tuomi, T.; Kasper, G. Toxicol. 2010, 269, 92.
Currently, there is a lack of information to quantify exposure to ENMs and the associated concerns. The basic transport and fate of nanoparticles from nanocomposites when exposed to different conditions are not well understood nor are their effects on biological systems and the environment. A research strategy report recently issued by the National Research Council in the U.S. stresses the need to assess the risk associated with exposure to ENMs, including modeling the fate and transport of nanoparticles. See for example, NRC (National Research Council) A Research Strategy for Environmental, Health, and Safety Aspects of Engineered Nanomaterials; National Academy Press: Wash., D.C., 2012, EFSA (European Food Safety Authority) EFSA Journal 2009, 958, 1-39, and Johnston, J. M.; Lowry, M.; Beaulieu, S.; Bowles, E. State-of-the-Science Report on Predictive Models and Modeling Approaches for Characterizing and Evaluating Exposure to Nanomaterials. U.S. Environmental Protection Agency: Washington, D.C., 2010. EPA/600/R-10/129 (NTIS PB2011-105273).
Nanoclays, such as organically modified montmorillonite (o-MMT), are most widely used for nanocomposite applications in the packaging and automotive parts industries because of their natural abundance, high mechanical strength, and high aspect ratio. See for example, Marquis, D. M.; Guillaume, É.; Chivas-Joly, C. In Nanocomposites and Polymers with Analytical Methods; 2005; pp. 261-284 and Jiang, T.; Wang, Y.; Yeh, J.; Fan, Z. Eur Polym J 2005, 41, 459-466. The good efficiency-cost balance of o-MMT as a nanofiller accounts for its use in about half of the entire nanocomposite market (approximately 60,000 metric tons in 2011). When o-MMT is compounded with polymers and exposed to moderate temperatures, these nanoparticles can move within the polymer matrix towards the surface and migrate to the surroundings. Non-diffusive mechanisms for nanoclay particle migration have been proposed to explain increases in the o-MMT content of nanocomposite surfaces during heating of polypropylene (PP)/o-MMT and nylon-6/o-MMT. See for example, Lewin, M. Fire Mater 2003, 27, 1-7, Zammarano, M.; Gilman, J. W.; Nyden, M.; Pearce, E. M.; Lewin, M. Macromol Rapid Comm 2006, 27, 693-696, Tang, Y.; Lewin, M.; Pearce, E. M. Macromol Rapid Comm 2006, 27, 1545-1549, Tang, Y.; Lewin, M. Polym Degrad Stabil 2007, 92, 53-60, and Lewin, M.; Tang, Y. Macromolecules 2008, 13-17. The movement of nanoclay particles could also be modified by other factors such as interaction with different solvents and radiation. A better understanding of migration in nanocomposites is extremely important for determining exposure dose, and this requires knowledge of the basic mass transport parameters of the nanoparticles.
Challenges in evaluating the transport and fate of ENMs from nanocomposites include the lack of tools and methodologies available to adequately track their movement and position. See for example, EFSA (European Food Safety Authority) EFSA Journal 2011, 9, 2140. The current approaches for tracking and detecting nanoclays involve elemental analysis via atomic absorption spectrometry (AAS) or inductively coupled plasma mass spectroscopy (ICP-MS) to detect trace amounts of a specific element. See for example, Avella, M.; De Vlieger, J. J.; Errico, M. E.; Fischer, S.; Vacca, P.; Volpe, M. G. Food Chem 2005, 93, 467-474, Schmidt, B.; Petersen, J. H.; Bender Koch, C.; Plackett, D.; Johansen, N. R.; Katiyar, V.; Larsen, E. H. Food additives & contaminants. Part A, Chemistry, analysis, control, exposure & risk assessment 2009, 26, 1619-27, and Schmidt, B.; Katiyar, V.; Plackett, D.; Larsen, E. H.; Gerds, N.; Koch, C. B.; Petersen, J. H. Food addit contam A 2011, 28, 956-966. However, these methods lack the ability to track single or clustered nanoclay particles and their positions, which hampers monitoring them in time, a key aspect in modeling the transport processes.
Fluorescent labeling is a promising approach for particle tracking due to its simplicity and inherently low detection limits. See for example, Dahan, M.; Alivisatos, P.; Parak, W. J. In Single Particle Tracking and Single Molecule Energy Transfer; Wiley-VCH Verlag GmbH & Co. KGαA, 2009; pp. 67-96. In nanocomposites, fluorescent labels have been used to monitor nanofiller homogeneity and to characterize colloidal stability in liquids and transport. See for example, Raccurt, O.; Samuel, J.; Poncelet, O.; Szenknect, S.; Tardif, F. In NSTI-Nanotech; 2008; pp. 704-707. Direct incorporation of a fluorescent organic dye into layered silicates like MMT can be accomplished by ionic exchange. This approach has been used to monitor the mixing and exfoliation processes during extrusion of polymer clay nanocomposites. See for example, Maupin, P. H.; Gilman, J. W.; Harris, R. H.; Bellayer, S.; Bur, A. J.; Roth, S. C.; Murariu, M.; Morgan, A. B.; Harris, J. D. Macromol Rapid Comm 2004, 25, 788-792. However, the fluorescent component is not adequately coupled to the clay substrate and could easily be dislodged from the substrate during the extrusion process.
Accordingly, a new method of attaching the fluorescent component to the substrate is necessary to provide stability to the bond between the fluorescent tag and the clay.